When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, modulus, tangent delta (tan δ), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, and improved fuel economy for vehicles equipped with such tires.
Mixing silica into rubber stocks, however, is difficult because silica particles containing polar silanol groups on the surface, tend to self-associate and reagglomerate extensively after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations.
To alleviate this problem, various silica coupling agents including, but not limited to, the well known bis(trialkoxysilylorgano) polysulfides (e.g., tetrasulfides and disulfides) and combinations of octyltriethoxysilane and mercaptoalkyltrialkoxysilanes have been employed to improve silica dispersion and compound viscosity. These coupling agents have a moiety (e.g., an alkoxysilyl group) that is reactive with the silica surface and a moiety (e.g., a mercapto or another sulfur group) that binds to the polymer.
Organoalkoxysilane compounds have also been employed as agents that react with the silica surface as shielding or hydrophobating agents to improve dispersion and compound viscosity. The alkoxysilyl groups of these compounds react with the silica surface but do not have a moiety that binds to the polymer. Well-known examples of these agents include, but are not limited to, alkyltrialkoxysilanes such as octyltriethoxysilanes, decyltriethoxysilanes, dodecyltriethoxysilanes, and their trimethoxysilane counterparts, and the like. In addition, it is known to terminate elastomers with a functional group that contains a silica-reactive alkoxysilane group to improve compound properties.
A feature of all of the aforementioned silica dispersing agents and functionalized elastomers is the presence of one or more alkoxysilane groups that react with the silanol groups on the silica surface (the alkoxysilane-silica reaction) during mixing of the rubber compound, with the evolution and release of alcohol into the environment. In particular, when the mixing is conducted at high processing temperatures, alcohol is released and contributes to the volatile organic compounds (VOCs) generated and potentially released during processing of the rubber compounds. At lower processing temperatures, the compounded product can retain a considerable amount of unreacted alkoxysilyl groups that are available to further react with the silica and moisture during storage, extrusion, tire build, and/or curing, resulting in an undesirable increase in the compound viscosity, and a shorter shelf life. This continuing reaction in the compounded product evolves additional alcohol which may impair further processing of the compound. As a result, a low tread strip drawing speed must be maintained to ensure the drawn product conforms to specifications, resulting in a decrease in production and a concomitant increase in costs.
As the present trend in rubber-making technology continues toward the use of higher silica loadings in rubber compounds, there is a challenge to contain levels of environmentally released alcohol. In addition, there is a need to reduce the amount of alcohol retained in the compounded product, in order to increase production and decrease costs. Therefore, a need exists to significantly reduce or eliminate the evolution of alcohol during compounding, processing, cure and storage of silica-reinforced rubbers.